The art of photolithography embodies techniques for creating patterns on a work surface by the controlled application of energy (such as electromagnetic, ion beam or other radiation) to a reactive material deposited on a wafer. The energy application is controlled through the use of a photomask. Photomask features are essentially a pattern of optically clear and optically opaque areas or fields formed on a glass plate. The optically opaque areas of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass, thereby creating light areas. Dark-field (or negative polarity) masks expose the contacts, while clear-field (or positive polarity) masks expose the lines. Optical effects, such as diffraction and convergence, directly affect device quality and must be addressed when manufacturing photomasks. One common problem is the edges between the transmissive and opaque areas cause diffraction, creating constructive interference, which in turn limits feature resolution. Constructive interference is caused by energy waves which are bent and re-radiated, resulting in exposure reduction in transparent areas and undesirable illumination in opaque or dark areas.
Diffraction is a significant limiting factor for optical photolithography. As manufacturing requirements call for increased feature densities and devices with smaller dimensions, the need for well-defined features and good image contrast increases as well. Increased pattern density on the mask decreases distance between any two opaque areas, as well as the overall size of each of the areas. As area feature size decreases, the impact of diffraction, as well as other optical effects, increases proportionately. In addition, conventional methods of controlling diffraction mandate a minimum distance between features, thereby decreasing process latitudes. One approach toward enhancing the performance of photo lithographic processing has been to employ phase-shifting technology as part of the masking material.
Phase-shift lithography shifts the phase of one area of incident energy waves approximately 180.degree. relative to an adjacent area of incident light waves to create a more sharply defined interface between the adjacent areas than is otherwise possible. The energy that would be diffracted is converted from constructive interference (proximity effect) to destructive interfering wave energy. A primary result of phase-shifting masks is improved image resolution and contrast. In general, a phase-shifting photomask is constructed with a repetitive pattern formed of three distinct layers. An opaque layer provides areas that allow no light transmission, a transparent layer provides areas which allow close to 100% of light to pass through and a phase-shifting layer provides areas which allow close to 100% of light to pass through but phase-shifted 180 degrees from the light passing through the transparent areas. The transparent areas and phase-shifting areas are situated such that light rays diffracted through each of the areas is canceled out, producing a darkened area. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the opaque layer of the mask on a photo-patterned semiconductor wafer. Phase-shifting masks have been shown to improve the resolution of photolithography between 25-100%.
There are several variations of phase-shifting masks, such as alternate aperture, rim and attenuated. Alternate aperture phase-shifting masks are formed by depositing a phase-shift material over the opaque layer and into every other opening in the opaque layer. This is termed an "additive" phase-shifting mask. Alternately, phase-shift areas of the mask may be formed in areas of the plate having a decreased thickness. This is termed a "subtractive" phase-shifting mask. This type of phase-shifting mask is patterned to ensure that light from transparent areas near enough to each other for the energy waves to interfere have opposite phases. The resulting image has a dark line, produced by destructive interference, between images of every transparent area. The images have as a result higher contrast, which translates to better feature resolution. Alternate aperture phase-shifting creates strong proximity effects, however, making it difficult to delineate all feature sizes and shapes for an arbitrary mask pattern using one common exposure.
In a rim phase-shifting mask, a layer of a transparent phase-shift material is deposited over the opaque areas and into the edge of the transparent areas. This produces rim phase-shifters on the sidewalls of the opaque areas on either side of each transparent opening. When using a rim phase-shifting mask, the light passing through a rim phase-shifter is phase-shifted relative to the light passing through the transparent area. As a result the phase-shifted light forms a null on the work surface that corresponds to the edges of the opaque areas. This creates destructive interference, negating the effect of diffraction along the edges of the opaque areas, producing sharpened images. Rim phase-shifting mask technology is limited in application, however, because custom fabrication processes are generally required to form the opaque layer to a minimum required thickness. Thus it tends to be an expensive and time consuming process. Standard mask blanks employed in semiconductor manufacture are difficult to use with this method because the opaque layer on these masks is not formed to the required thickness. In addition, a rim phase-shifting mask requires more mask area than other techniques because it requires large bias to reduce exposure times to a reasonable level.
Both alternating aperture and rim phase-shifting techniques increase the complexity of mask fabrication, in part as a result of being three-layer structures. Attenuated phase-shifting masks have the advantage of being a two layer structure, simplifying the manufacturing process. Attenuated phase-shifting masks, described in Burn J. Lin, "The Attenuated Phase-Shift Mask", 43-47 Solid State Technology (January 1992), use a slightly transmissive absorber with a 180.degree. phase-shift in the place of the opaque material in the mask pattern. Unlike many other phase-shifting masks, attenuated phase-shifting masks can be used for any arbitrary mask pattern. An attenuated phase-shifting mask shifts the phase of dark areas but with an attenuated amplitude to prevent producing too much light in those areas. The negative amplitude provides the desired improvement in image edge contrast and the attenuation prevents the negative amplitude from becoming a problem. Resolution of closely packed features is further improved by using an attenuated phase-shifting mask with a mask bias because exposure times and diffractive effects can be reduced. Conventional attenuated phase-shifting dark-field masks use a positive mask bias, and clear-field masks use a negative bias. In both mask types openings are larger and opaque features are smaller on the mask than the size printed on the wafer. As an example, Lin, in U.S. Pat. No. 5,288,569 ("Feature Biasing and Absorptive Phase-Shifting Techniques to Improve Optical Projection Imaging") describes a system for improving pattern processing by using, in conjunction with absorptive phase-shifting masks, a negative bias for isolated or less dense arrangements of features. The bias helps control the effect of side lobe light resulting from diffraction at the feature edges, but the bias also takes a toll on process latitudes.
Alternate aperture and rim phase shifting masks have the disadvantage of requiring a significant mask area between mask patterns. This makes it difficult to reduce the distance between adjacent patterns on the work surface and thus limits feature density. While attenuated phase shifting masks provide the ability to reduce the inter-pattern space, interference of side-lobe light from masked areas with the main lobe light of the pattern areas results from the difference in phase between the respective areas. This reduces the effect of the phase shift and detrimentally effects the intended pattern. What is needed is a way to achieve the positive effects of attenuated phase shifting while avoiding or preventing side lobe interference without severely impacting process latitudes. One partial solution is described in U.S. Pat. No. 5,487, 963, entitled "Phase Shifting Mask Wherein Light Transmitted Through Second Transmission Areas Intensifies Light Through First Transmission Areas", and granted to Sugawara, wherein side lobe light is dealt with by turning it to an advantage. Sugawara discloses a method for forming dense patterns such that side lobe light from one opening enhances the main lobe light of an adjacent opening. The combination of bright areas of enhanced main lobe light and dark areas between openings created by destructive interference through use of phase shifting material between openings enables the manufacture of a better-defined pattern. The method described by Sugawara is limited, however, because side lobe light from the phase-shifted areas destructively interferes with the adjoining bright areas, reducing the effect of the combined energy of adjacent patterns as feature size decreases. There remains a need for a means of reducing or eliminating the effect of side lobe interference without reducing process latitudes.
Another area of concern with conventional methods for processing photomasks using attenuating material in conjunction with phase-shifting techniques is the degradation of resist. As exposure times increase, resist thickness is degraded in large transmission areas as a result of the extended exposure. What is needed is a means for equalizing the exposure of transmission areas in an attenuated phase-shifting photomask.